You can promote healthy soil in your garden and cut down on plant disease by practicing a three-year cycle of crop rotation with eight plant families.
One of the rules of good organic gardening is to rotate plant families from one season to the next, as best you can, so related crops are not planted in the same spot more often than every three years or so. The purpose of crop rotation is to help maintain the balance of nutrients, organic matter, and microorganisms necessary for healthy soil. Of these three, the invisible world of soil-dwelling micro-creatures is the one that most benefits from crop rotations.
Take potatoes, for example. In the course of a season, the fungi that cause scabby skin patches may proliferate, along with root-killing verticillium fungi (which also damage tomatoes and eggplant) and tiny nematodes that injure potatoes. If you plant potatoes again in the same place, these pathogens will be ready and waiting to sabotage the crop. Rotating the space to another unrelated crop deprives the potato pathogens of the host plant they require. Most pests and diseases can damage plants of the same botanical family, but cannot hurt unrelated crops (see “Rotate Your Families: The Nine Main Groups,” below).
What if you don’t follow a crop rotation plan? Field trials in Connecticut and Europe indicate that your potato production will quickly fall by 40 percent, mostly due to disease. According to a seven-year study from Ontario, you could expect similar declines if you planted tomatoes in the same place over and over again. Compared to eight different rotations with other vegetables or cover crops, continuous tomatoes consistently produced the lowest yields. Snap beans that are not rotated will turn into paltry producers, too. In a recent study from Cornell University, snap bean production doubled when beans were planted after corn rather than after snap beans.
In addition to interrupting disease cycles, rotating crops prevents the depletion of nutrients. For example, tomatoes need plenty of calcium the same way beans and beets crave manganese. But the exact benefits of effective rotations vary with crop sequence. Broad-leafed greens are great for suppressing weeds, and the deep roots of sweet corn do a good job of penetrating compacted subsoil. Nitrogen-fixing legumes often take no more nitrogen from the soil than they replace, and their presence stimulates the growth of beneficial soil microorganisms. But in some situations, the “rotation effect” defies easy explanation. For example, we don’t know precisely why potatoes tend to grow well when planted after sweet corn, but they do.
The subject of crop rotation can get complicated so fast that it’s no wonder we are tempted to cheat. What if your garden is like mine — a collection of a dozen permanent beds that are planted with 20-plus different crops in the course of a growing season? Not using rotations would be unwise. When researchers at Pennsylvania State University tracked early blight of tomatoes grown in the same place for four years, early-season infection rates (measured when 5 percent of fruits turned red) went from 3 percent in the first year to 74 percent in the third. When they tried the same monoculture maneuver with cantaloupes, symptoms of alternaria blight appeared earlier and earlier with each passing season.
Some organic gardeners point out that crop rotation guidelines developed for farmers don’t really fit home gardens. On farms, crop residue is either plowed under or left on the surface to decay, which means the soil receives large infusions of a single type of plant material. Gardeners are more likely to pull up and compost spent crops, and to dig in compost or other soil amendments between plantings, which replenishes nutrients and invigorates the soil food web in an extremely diversified way. Biodegradable mulches introduce more considerations: If you heavily mulch your potatoes with straw, shredded leaves, grass clippings or all three, certainly it makes sense to factor those forms of organic matter into your rotation plans.
But don’t think that just because you pull up plants you are interrupting the food supply of soilborne plant pathogens to the point that you can ignore rotations. When I pull up beans, for example, only a small tangle of roots comes up with the plants; the bulk of the root system stays behind in the soil. If I plant beans in that row again within two years, the plants will be at risk for micronutrient deficiency and several major bean diseases. Using a three-year crop rotation radically reduces the chances that my beans will be bothered by root rot, white mold and several serious blights . This is an excellent reward for taking on the task of devising a workable rotation plan in which new plantings are helped along by the bed’s previous tenants.
The eight-crop rotation plan developed by market gardener Eliot Coleman incorporates decades of farm and garden research, and it’s a great place to start planning rotations for your garden. In order, Coleman’s plants unfold like this: (1) tomatoes (2) peas (3) cabbage (4) sweet corn (5) potatoes (6) squash (7) root crops (8) beans. If you grow only these eight crops in eight rows or beds, you now have your rotation plan. Simply line up your crops in the right order, and shift them one space over every year.
But it’s not likely to be this simple for your garden, so you will need a customized plan that relocates the main plant families from one season to the next. (Families are crops that are closely related and therefore prone to many of the same pests and diseases.) The nine plant families grown in vegetable gardens are summarized above, but expect to need more space for some families than others. For example, you may need a lot of space for tomato-family crops (tomatoes, peppers, potatoes) and only a little for spinach, chard, or beets, and you may not be able to grow space-hungry sweet corn at all. Begin planning your rotations by making a list of your must-have crops and how much space is required by each one. Then sort them into the plant families.
You will also need to identify “crop sequences” that work well in your garden within the same growing season. For example, many gardeners have garlic in the ground from fall to midsummer, after which the area can be planted with a second crop. In my Zone 6 garden, I can grow shell beans after garlic if I hustle, which gives me a garlic/bean sequence. In a cooler climate, you might have a garlic/lettuce sequence.
Other sequences that work well for me include a snap pea/carrot sequence, an onion/leafy green sequence, and a broccoli/bush bean sequence. Add any crop sequences you often use (or want to try) to your list.
You will need two sheets of paper, scissors, and a pen or pencil. On one sheet, make a rough drawing of your garden, noting the sizes of beds or rows. Write down, to the best of your knowledge, where various crops grew last year. If you take photographs of your garden at different times during the season, it’s much easier to recall where you planted what.
Cut another piece of paper into smaller pieces that fit the rows or beds in your garden drawing, and copy the crops, the plant family they belong to, and crop sequences from your list onto these “crop markers.” In my garden, I end up with markers for 10 crops or crop sequences to rotate within my 12 permanent beds. It’s good to have a couple of beds for trial plantings and irresistible whims.
Back at the drawing board, spend some time puzzling through your plans by moving the labeled crop/plant family markers about on your garden drawing. Your goal (which may take a few seasons to implement) will be to have your plantings move in a logical order and direction, whether it’s left to right, front to rear, circular or whatever. Expect to improvise and innovate. For example, I am now growing more peas and beans to stretch out years between onion and garlic plantings, which are priority crops in my garden. When in doubt about a rotation, I slip in a crop of beans or leafy greens.
If you feel frustrated, play with your drawing and markers for a while, and then pack them up for a few days of thinking time. When you go back to the task, you will probably have an easier time finding at least a few effective rotations waiting to be put into action. After a few years of fine-tuning, the payoff for this level of garden planning can be huge — a long-term rotation plan that runs itself and benefits every crop you grow.
You can simplify your rotations by sorting your crops into these major plant families: